It has recently been discovered that a 50-75 wt % blend of butene-1 homopolymer (PB) (with a melt index greater than 100 and less than 1000), about 23.5 to 39.88% by weight of stabilized propylene polymer (PP) and 0.02 to 1.5 high density polyethylene may be used for the production of a molded article which has an excellent overall balance of properties including improved cycle time. Improved cycle time can be defined as a shorter time required for molding after extrusion than for usual types of blends.
In U.S. Pat. No. 3,733,373 to McConnell et al., the invention is directed to improving the usefulness of butene-1 homo- or copolymers as molding resins. In molding these resins, the polymer crystallizes out of the melt as a soft, clear plastic (Form II polybutylene crystallization) and then gradually changes to the more thermodynamically stable Form I. The patentees state that during this transformation the molded objects are tacky and warp and become cloudy, and that the molded object tends to be so soft that the knock-out pins of the mold tend to puncture the molded parts. The patentees disclose that addition of high or medium density polyethylene substantially increases the crystallization temperature of the poly-1-butene and permits the blends to mold extremely well and to be released from the mold without being punctured or dented by the knock-out pins. A concentration range of 0.1-30% by weight of ethylene polymer is disclosed as useful. The increase in crystallization point, measured by Differential Scanning Calorimetry (DSC) is used in the examples as a measure of effectiveness of the additive. However, the addition of polypropylene to the blends of the patent is not disclosed.
U.S. Pat. No. 3,455,871 issued to Coover, assigned to Eastman Kodak Co., describes substantially crystalline homopolymer and copolymers of 1-butene as a concentrate carrier for non-olefinic additives. The Coover patent does not claim or describe the same kind of material as used in the present invention. Coover claims a butene-1 material with an intrinsic viscosity in the range 0.2 to 5.0. The present invention has properties different from the Coover material.
The compositions according to this invention comprise a blend of about 60-75% by weight of an isotactic butene-1 homopolymer with a melt index greater than 100 and less than 1000, about 23.5-39.8% by weight of stabilized isotactic propylene, and about 0.05 to 1.5% by weight of high density polyethylene and a small but critical amount of a mold release agent to provide a material with improved cycle time, up to 20% improvement (i.e. at least 20% reduction in the cycle time) as compared with known art.